The present disclosure relates to semiconductor wafer structures suitable for laser processing for separating into individual semiconductor devices.
Conventionally, blade dicing has been the most generally used method of dicing semiconductor wafers. In the blade dicing, a ring-shaped dicing saw, which holds particles of diamond and cubic boron nitride (CBN) using a bonding material, is rotated at high speed to break a wafer along a dicing lane (the actual dicing width of the dicing saw) as an area needed for separating the wafer.
In a dicing technique using a dicing saw, processing quality has been improved by improving and optimizing a specification of the dicing saw such as a size and density of a diamond particle, and a bonding material; and an operational condition such as rotation speed, feed speed, and cutting depth.
However, there are limits on improvements in quality of processing with a dicing saw. Further improvements cannot be expected in breaking wafers with a dicing saw, particularly because of the following problems.
(1) The breaking causes chipping on a cutting plane to reduce breaking strength of semiconductor devices after dicing.
(2) A fragment of chipping reduces as dust, process yields and reliability of the devices after dicing.
(3) The dicing saw generally needs to have a thickness of 20 μm or more to maintain the mechanical strength. A scribe region needs to be larger than the actual dicing width so that the chipping does not enter the region for the semiconductor elements.
(4) Water is used during the processing such as cooling a wafer to reduce heat generation caused by breaking, and washing the wafer to remove dicing dust. Thus, the dicing technique using a saw cannot be used for a water sensitive device such as a micro electro mechanical system (MEMS).
In recent years, as solution to the above problems, much attention has been paid to processing with laser light. For example, Japanese Patent Publication No. 2002-192370 describes a technique for forming a modified region in an object by multiphoton absorption. The multiphoton absorption is the phenomenon, in which light absorption occurs in a material because of a significant increase in intensity of light, even when energy of photons is smaller than the band gap of the light absorption, i.e., when the material is optically transparent. In this method, laser light is focused on the inside of the semiconductor wafer to cause multiphoton absorption, thereby forming the modified region inside the semiconductor wafer. Then, a crack is grown along a predetermined separation line from the modified region as a starting point to separate the semiconductor wafer. This enables dicing of the semiconductor wafer without generating any undesired crack, i.e., chipping, outside the predetermined separation line. Therefore, the conventional method reduces the dust, and the breaking strength caused by chipping. Unlike a breaking technique, dicing with laser light does not require physical cutting width in a planar direction. This leads to significant reduction in the area of the dicing region. Furthermore, water is not required, since the dicing does not cause dust and heat generation in the processing. Therefore, the dicing is suitable for processing of a water sensitive device.
In the case of a thick semiconductor wafer as described in Japanese Patent Publication No. 2002-205180, the depth of the focal point is changed to form a plurality of modified regions at various depths in the semiconductor wafer. Cracks generated from the respective modified regions are connected to each other, thereby enabling the separation of the wafer. At this time, with an increase in the thickness of the semiconductor wafer, an increasing number of modified regions are needed, thereby requiring more time for processing. When distances between the modified regions are set long to reduce the number of the modified regions, or when there is a long distance from the modified regions to a surface of the semiconductor wafer; reliable separation cannot be expected and non-separated parts are formed. Even if separation is performed, propagation linearity of the cracks are degraded. This results in deterioration of propagation linearity in the semiconductor surface.
Japanese Patent Publication No. 2003-88980, for example, describes a method of growing cracks with fewer modified regions to reliably separate a semiconductor wafer. In this publication, after forming the modified regions, the semiconductor wafer is cooled, and thermal stress is applied to the wafer to grow the cracks of the modified regions. Japanese Patent Publication No. 2005-268752, for example, describes a method of improving propagation linearity of cracks. In the method, a surface of a semiconductor wafer is scratched to form a recess, and a crack from a modified region is guided to the recess to enable separation with propagation linearity.